1. Field of the Invention
The present invention relates to micromachined devices and, more particularly, to a suspension system for micromachined devices.
2. Description of Related Art
Recent advances in micromachining have enabled the manufacture of various microelectromechanical systems (MEMS) that offer potential performance and cost improvements over existing non-micromachined devices. MEMS devices may be manufactured on a large scale using photolithographic techniques to etch silicon wafers, in much the same way that traditional microelectronic integrated circuits are produced in the electronics industry. In silicon-based MEMS devices fabricated using conventional integrated circuit techniques, three-dimensional structures can be integrated with electronic circuitry on the same chip, offering great potential for improvements of sensors, actuators, and other devices. Initially, MEMS devices were strictly silicon-based, like microelectronic devices, but today the term represents complete miniature devices that may or may not be silicon-based, and that can be produced using methods other than photolithographic techniques.
One MEMS device is a micro-electromechanical system gyroscope (MEMS gyro). The MEMS gyro consists of one or more oscillating proof masses that may be suspended above a substrate by spring elements mounted to the substrate. The proof mass is made to oscillate at a precise frequency axially and parallel to the substrate by an electronic drive mechanism. As used herein, the term xe2x80x9cproof massxe2x80x9d is defined broadly to include any mass suitable for use in a MEMS system. The MEMS gyro functions by sensing the coriolis acceleration that acts on the oscillating proof mass when the gyro is rotated. Further, the substrate typically has a recess below the proof mass that allows the gyro to oscillate freely above the substrate. The recess may be formed in the substrate by deposition of a photoresist mask that allows the substrate to be selectively etched.
When spring elements are used to suspend a proof mass above a substrate, at least one end of the spring element is typically mounted to the substrate, and the other end is typically attached to the proof mass. Because one end is fixed, and also because micro-machined structures do not have pin joints, a spring element must typically stretch as well as bend when the proof mass oscillates axially. Adding spring elasticity to each of the spring elements used to suspend a proof mass can accommodate this stretching.
When proof masses are mounted so that their spring elements must stretch to allow movement, however, the resulting spring constants in the direction of oscillation are non-linear. Non-linear spring constants can introduce frequency shifts if the amplitude of the mass"" oscillation varies. Such frequency shifts are undesirable, as they can affect the accuracy of a MEMS gyroscope. Moreover, the performance of any micromachined device that employs a movable mass may be adversely affected by a non-linear spring constant in the suspension system. Thus, a suspension system with a more linear spring constant could provide improved performance in micromachined devices.
In addition, suspending a proof mass with a spring element that is configured to stretch and also to bend as the proof mass oscillates allows some freedom of motion in directions other than the direction in which the proof mass was designed to oscillate. Such freedom of motion is undesirable, can adversely affect measurements made by the gyro and, if it is great enough, may even damage or destroy the gyro if a portion of the proof mass collides with a stationary element of the gyro. Thus, a suspension system that allows great freedom of motion along one axis, while significantly restricting motion in any other direction in the plane of the substrate may provide improved reliability and performance in micromachined devices.
A suspended micromachined structure is disclosed. The structure may include a movable proof mass, and multiple support arms configured to suspend the proof mass above a substrate. Each support arm may include one or more spring elements and at least one rigid lateral element.
Preferably, the proof mass is integrally connected to a rigid lateral element of each of four support arms suspending the proof mass above the substrate. Each support arm preferably includes a rigid lateral element having spring elements extending therefrom. The spring elements may in turn be attached to the substrate at two points, and each rigid lateral element of the support arms may be attached to the proof mass at one point. Preferably, the spring elements are attached to opposite ends of each rigid lateral element. In addition, the proof mass is preferably attached to the center of the rigid lateral element. Thus, there are preferably three points of attachment on the rigid lateral element. These points of attachment create three effective flexure points or flexure points for each support arm along each rigid lateral element. Thus, there is a flexure point at the intersection of the proof mass and the rigid lateral element, as well as at the intersection of the rigid lateral elements with the respective spring elements.
The effective flexure points of the support arms may be configured to allow the proof mass to move with a great deal of freedom axially, parallel to the substrate, while allowing substantially less freedom of movement in any other direction within a plane parallel to the substrate. Further, the support arms may be configured so that the spring constants that act on the proof mass result from bending of the spring elements, greatly improving the linearity of the net spring constant of the suspension system. In such a system, the flexure point associated with the point of attachment between the proof mass and the rigid lateral element moves in a linear direction parallel to the axial direction of motion of the proof mass, allowing for improved performance of the gyro.
These as well as other aspects and advantages of the present invention will become apparent to those of ordinary skill in the art by reading the following detailed description, with appropriate reference to the accompanying drawings.